The present invention relates to industrial circuit breakers and particularly to a trip initiating thermal override for molded case industrial circuit breakers equipped with static (solid state) trip units.
It is known that a molded case circuit breaker can develop an excessive internal ambient temperature condition even when conducting current well within its current rating. This overheating condition is caused by abnormal losses in the breaker current paths typically occasioned by poor electrical connections or deteriorated breaker contacts.
Circuit breakers equipped with traditional thermal or thermal-magnetic trip units are afforded a reasonable measure of protection against a self-destructive thermal runaway condition since the current heating of the thermally responsive trip element, typically a bimetal, subjected to a rising ambient temperature environment will eventually precipitate tripping of the circuit breaker. Unfortunately, static trip units, now being increasingly implemented in industrial molded case circuit breakers, only respond to the currents flowing in the breaker poles and thus, unlike thermal and thermal-magnetic trip units, are not normally responsive to internal ambient temperature. To afford ambient temperature responsiveness, it is known to equip a static trip unit with a temperature sensor, such as a thermistor, operative to produce a trip signal when overheating occurs. However, this approach to thermal protection relies on the continued operability of the static trip unit whose electronic components are particularly susceptible to damage by high ambient temperatures. Thus, any appreciable time lag in the temperature sensor's response can render the static trip unit totally inoperative to initiate tripping of the circuit breaker.
It is accordingly an object of the present invention to provide a direct acting thermal trip override for static trip circuit breakers.
A further object is to provide a thermal trip override of the above character which utilizes a temperature responsive element operating independently of the trip unit electronics to reliably initiate tripping of a circuit breaker automatically in response to the ambient temperature within the breaker enclosure exceeding a safe level.
An additional object is to provide a thermal trip override of the above character wherein the temperature responsive element is incorporated in and operative to initiate thermal override circuit breaker tripping action of an electromechanical, flux shifting trip device otherwise electrically activated under the control of a static trip unit to initiate a circuit protective trip function.
Other objects of the invention will in part be obvious and in part appear hereinafter.